Influence of the c-axis orientation on the optical properties of thin CdS thin films formed by laser ablation

Thin (approximately equals 1.5 micrometer) CdS films were prepared on glass by laser ablation using fluences of 2 - 5 Jcm-2. We demonstrate that such an increase of the laser fluence turns the orientation of the c-axis of the films from perpendicular to parallel with respect to the substrate surface. The influence of this orientation variation on the optical properties of the films is studied by photocurrent, transmission and z-scan measurements. All experiments were carried out at 300 K using monochromatic light or the cw emission of argon and He-Ne lasers at 514.5 and 632.8 nm, respectively. The transmission threshold and the photocurrent maxima are shifted to shorter wavelengths and the transmission edge becomes steeper with increasing the laser fluence. The nonlinear absorption and refraction indices were evaluated for 514.5 nm and 632.8 nm by z-scan technique. It occurred that at 514.5 nm the photo-thermal heating due to effective absorption dominates and, therefore, refractive nonlinearities are not provable. At 632.8 nm, however, the samples are transmissive and refractive nonlinearities are clearly observed. Higher nonlinear coefficients of absorption and refraction were found for samples with parallel c-axis. As far as we are aware, this work represents the first study of the influence of the crystal direction on the photocurrent and z-scan features of oriented thin CdS films.